Interactions of cells with their immediate extracellular matrix are thought to be critical in the control of cellular functions such as adhesion, replication, motility and differentiation. Cell surface and extracellular matrix proteins are thought to mediate these interactions, with fibronectin being one of the most thoroughly studied of these adhesive proteins. Recent work has identified a new protein oligomer, called "hexabrachion", as a major component of cell surface fibronectin preparations. At least one of the activities previously ascribed to cell surface fibronectin (hemagglutination) has been shown to be due entirely to this oligomer. Studies are proposed to delineate the role of the hexabrachion in the interaction of cells with their environment. (1) Adhesion assays will be used to determine whether the hexabrachion serves as one of the cell-substratum attachment molecules. Microtiter wells will be coated with hexabrachion, fibronectin, or both and tested for their ability to support adhesion of different cell types. (2) The effect of the hexabrachion on cell spreading and restoration of normal morphology to transformed cells will also be examined both by adding transformed cells to coated wells and by preincubating the cells in the presence of hexabrachion. (3) The types of cells that produce hexabrachions will be delineated by direct examination of culture media for hexabrachions, by enzyme linked immunoadsorbent assay (ELISA), and by SDS-polyacrylamide gel electrophoresis (SDS-PAGE) and immunobloting. The effect of cell culture aging on the production of hexabrachion will also be assessed. (4) Monoclonal and polyclonal antibodies to hexabrachion will be used to attempt to delineate the domains of cell adhesion and cell spreading activities and further define the mechanisms of interaction of the hexabrachion with the cell surface and with the other proteins which comprise the extracellular matrix.